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ELECTRICAL EQUIPMENT 



FIG. 211. Vertical Generator with Direct-connected Exciter. 



reserve capacity. Occasionally three units are installed, two of 

 which, combined, can take care of the entire excitation, the third 

 unit being the reserve. This method may be the most desirable, 

 on account of the possibility of debris or ice clogging up the small 

 exciter turbines and shutting them down. Under such condi- 

 tions it would naturally be more advantageous to keep a motor- 

 driven set in reserve for such an emergency. From an economical 

 point of view, however, it is evident that two motor-driven units 

 with a spare water-driven unit would cost less. An objection to 

 motor-driven sets which is occasionally raised is that they are 

 liable to drop out of step when a short circuit occurs on the system. 

 This is, however, not the case with well-designed sets under 

 momentary short circuits, and where it has occurred, it has been 

 prevented by equipping the sets with flywheels. This, of course, 

 increases the expense of the sets and is, as a rule, not justified. 



